WPF: unable to get custom attached property to work as trigger property

Question

So, I have a class that looks like this:

internal class MyClass
{
    public static readonly DependencyProperty IsSomethingProperty =
            DependencyProperty.RegisterAttached(
                "IsSomething", // property name
                typeof(bool), // property type
                typeof(MyClass), // owner type
                new FrameworkPropertyMetadata(false)
                );

    public static void SetIsSomething(DependencyObject obj, bool value)
    {
        obj.SetValue(IsSomethingProperty, value);
    }

    [AttachedPropertyBrowsableForType(typeof(TreeViewItem))]        
    public static bool GetIsSomething(DependencyObject obj)
    {
        return (bool)obj.GetValue(IsSomethingProperty);
    }
}

I would like to be able to use this nested property as a trigger property in a control template, for example:

<ControlTemplate TargetType="TreeViewItem">
    <!-- stuff here omitted for brevity -->
    <Trigger Property="my:MyClass.IsSomething" Value="True">
        <!-- setters for when IsSomething is True -->
    </Trigger>
</ControlTemplate>

(the above control template assumes the correct one xmlns:my="clr-namespace:MyAssembly", where MyAssembly contains MyClass in the attached XAML file)

Here is my problem: when I do this, it compiles fine. However, when I try to see this control pattern in action in the constructor, it complains about Cannot find the 'IsSomething' template property on type 'System.Windows.Controls.TreeViewItem'., and the constructor does not load.

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